1. Field of the Invention
The present invention relates to a vacuum ultraviolet-excited ultraviolet phosphor and to a light-emitting device that use this phosphor.
2. Description of the Related Art
Light-emitting devices such as fluorescent lamps and displays have come into wide use. Among these devices are lamps that use mercury as an excitation source, but with the increasing awareness regarding the impact on the environment in recent years, there is now an intensive search for light-emitting devices that do not employ mercury as an excitation source.
In addition, ultraviolet light of wavelength 254 nm that is radiated by mercury has a strong effect on human eyes and skin, and light-emitting devices that employ mercury as an excitation source therefore necessitate measures such as shielding to prevent the escape of ultraviolet rays, and these measures have been an obstacle to the miniaturization of the light-emitting devices. Still further, the mercury vapor pressure in mercury lamps drops in cold atmosphere such as in frigid regions or during winter, and discharge is therefore difficult to initiate in such cases. Even when discharge is started, there is the problem that the rise in temperature after lighting results in an increase in the mercury vapor pressure inside the bulb, with the consequence that the light-emitting intensity varies from the time of lighting until a stable state is achieved. A mercury lamp therefore requires time until light output stabilizes and has a slow rise-time characteristic. For these reasons as well, light-emitting devices that do not employ mercury are now in great demand.
Discharge lamps that use a rare gas such as xenon are known as light-emitting devices that do not use mercury as an excitation source. Rare-gas discharge ultraviolet fluorescent lamps do not suffer from the above-described problems that arise when mercury is used as the excitation source and therefore have potential in applications that have been receiving attention in recent years such as in deodorizing devices that take advantage of photocatalytic reactions using titanium oxide and in trans-illuminators for use in DNA analysis. In addition, with the technological advances in recent years, displays that are made up of plasma display panels that use rare gas as an excitation source are now receiving attention as a replacement for devices that are constituted by color Braun-tubes.
Ultraviolet phosphors that are known for use in such light-emitting devices include SrB4O7: Eu, BaSi2O5: Pb, YPO4: Ce and LaPO4: Ce. In addition, Japanese Patent Laid-Open Publication No. 081460/2001 and Japanese Patent Laid-Open Publication No. 172624/2001 disclose ultraviolet phosphors that contain gadolinium (Gd). Japanese Patent Laid-Open Publication No. 348571/2002 further discloses a vacuum ultraviolet-excited ultraviolet phosphor that is composed of a gadolinium-activated rare-earth aluminoborate that is represented by the general formula (Y1-xGdx)Al3(BO3)4 (where 0<x≦1).
It is known that phosphors that are composed of, for example, SrB4O7: Eu, BaSi2O5: Pb, YPO4: Ce and LaPO4: Ce that have been used in the prior art emit light by excitation by ultraviolet light having a wavelength of 254 nm that is emitted by mercury, but the intensity of light emission by excitation by vacuum ultraviolet light (VUV) of wavelengths below 200 nm is inadequate. Thus, when these phosphors are applied to light-emitting devices that use a rare gas as an excitation source, adequate light emission characteristics cannot be obtained.
Finally, although the vacuum ultraviolet light-excited ultraviolet phosphor that is composed of the gadolinium activated rare-earth aluminoborate that is represented by the general formula (Y1-xGdx)Al3(BO3)4 (where 0<x≦1) that is disclosed in Japanese Patent Laid-Open Publication No. 348571/2002 features excellent ultraviolet light emission intensity when compared to phosphors composed of the above-described SrB4O7: Eu, BaSi2O5: Pb, YPO4: Ce and LaPO4: Ce, these light-emission characteristics are still unsatisfactory when applied to various light-emitting devices. The market therefore still demands a phosphor that has still greater light-emitting intensity and can deliver ultraviolet light at higher efficiency.